The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising maristone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale after kerogen decomposition contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423 issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a cavity containing a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. The cavity has bottom, top, and side boundaries of unfragmented formation. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing "retorted oil shale".
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a heated zone such as a combustion zone and/or a retorting zone in the retort and introduction of an oxygen containing inlet gas such as air into the combustion zone to advance the combustion zone through the fragmented mass in the retort. The combustion zone can be established in the fragmented mass by burning a hydrocarbon containing gas, liquid and/or solid in the presence of air. In the combustion zone, oxygen in the inlet processing gas is depleted by reaction with hot residual carbonaceous materials to produce spent or dekerogenated oil shale and heat. By the continued introduction of the oxygen containing inlet gas into the combustion zone, the combustion zone is advanced through the retort.
Hot effluent gas from the combustion zone passes through the retort on the advancing side of the combustion zone to heat oil shale in a retorting zone in the fragmented mass to a temperature sufficient to produce thermal decomposition of kerogen, called retorting, in the oil shale to gaseous and liquid products and a residual product of solid carbonaceous material. Heat of combustion is carried from the combustion zone to the retorting zone largely by gas flow. Thermal decomposition of kerogen in the oil shale proceeds at about 800.degree. F., and appreciable quantities of carbonaceous materials are driven off from the oil shale at even lower temperatures. It will be recognized that the rate of progression of the combustion zone is quite slow and is ordinarily in the order of only a few feet per day. The combustion zone is not a thin layer but ordinarily has appreciable thickness due to gradual consumption of oxygen in the downwardly flowing gas and inherent variations in particle size of the oil shale. The combustion zone is the portion of the retort where the greater part of the oxygen in the combustion feed that reacts with residual carbonaceous material in retorted oil shale is consumed.
It is found that the best yield of shale oil from oil shale is obtained when the combustion zone moves downwardly through the retort as a substantially flat horizontal zone. If all or part of the combustion zone is skewed from the horizontal and/or warped, there is some tendency for it to approach the horizontal, but this is slow and the combustion zone may progress a substantial distance through the retort before this occurs. When the combustion zone is skewed some of the shale oil produced may be burned, thereby reducing the total yield. In addition, with a skewed and/or warped combustion zone, excessive cracking of hydrocarbon products produced in the retorting zone can result. It is, therefore, desirable to have the combustion zone progress downwardly through the retort as a substantially flat horizontal wave.
Establishment of a combustion zone in the retort can be effected according to the method described in U.S. Pat. No. 3,990,835, issued Nov. 9, 1976, and U.S. Pat. No. 3,952,801 issued Apr. 27, 1976, both of which were issued to Robert S. Burton III and assigned to the assignee of this application. Both of these patents are incorporated herein by this reference. The '801 patent describes a technique for establishing a combustion zone in a retort by igniting the top of a fragmented permeable mass in the retort. According to this technique, a hole is bored to the top of the fragmented permeable mass and a burner is lowered through the bore hole to the oil shale to be ignited. A mixture of a combustible fuel such as LPG (liquefied petroleum gas) and gas containing oxgyen, such as air, is burned in the burner and the resultant flame is directed downwardly towards the fragmented permeable mass. The burning is conducted until a substantial portion of the oil shale has been heated above the self-ignition temperature of carbonaceous material in the oil shale so combustion of oil shale in the fragmented mass is self-sustaining. Then introduction of fuel is terminated, the burner is withdrawn from the retort through the hole, and oxygen supplying gas is introduced to the retort to advance the combustion zone through the retort.
An in situ oil shale retort may have a substantial lateral extent; for example, it may be square with a width of 100 ft. or more. Ignition of the top of the rubble pile in a completely filled retort requires access which is ordinarily obtained by forming a conduit through the overlying unfragmented rock. In a relatively smaller retort a single central conduit may be used. In a larger retort a number of conduits to various top portions of the retort may be preferred. These conduits supply combustion air or other oxidizing gas during normal operation of the retort and their openings into the retort are also used to locate points of ignition for establishing a combustion zone. Since the ignition points are isolated, the top of the retort is inherently nonuniformly ignited.
For purposes of exposition a single ignition point in the center of a retort can be assumed. A retort with a number of separate ignition points can be considered as a plurality of adjacent smaller retorts, each with a single ignition point. Ignition is obtained by burning a combustible gas with air or other oxygen supplying gas and impinging the flame on the top of the rubble pile at the opening of the conduit. This heating may be conducted for a substantial period of time so that a sufficient volume of rock is heated to sustain combustion after the initial burning is stopped and air or the like is forced down the conduit. The combustion zone that is formed around the ignition point tends to progress downwardly and outwardly. It is driven downwardly by the gas flowing through the retort and progresses laterally primarily by conduction and radiation which are much slower. Substantial unburned portions may be left in the upper "corners" or side edges of the retort. Self-ignition temperature of the carbonaceous material in oil shale can vary with various conditions such as total gas pressure and the partial pressure of oxygen in the retort, and may be as low as 500.degree. F., although 750.degree. F. is usually considered a minimum. In operation of an in situ retort it is preferred to consider 900.degree. F. as the self-ignition temperature. Temperatures in the combustion zone of a retort may be 1200.degree. F. or more.
Since nonuniform ignition results in a nonhorizontal and non-planar combustion zone travelling through the retort, it is desirable to provide a technique for establishing and maintaining a combustion zone in an in situ oil shale retort where the combustion zone is flat and uniformly transverse to its direction of advancement and extends laterally to the boundaries of the retort.
The combustion and retorting zones are advanced through the fragmented permeable mass in the retort until near or at the end of the fragmented mass. Cooling by cold gas or air introduced into the retort on the trailing side of the combustion zone forms a cooling zone on the trailing side of the combustion zone. Cooling below the ignition temperature and depletion of carbonaceous material in spent shale on the trailing side of the combustion zone can cause discontinuance of combustion on the trailing side of the combustion zone. As used herein, the term "heated zone" refers to a hot portion of the fragmented mass such as a combustion zone and/or a retorting zone. Further, as retorting proceeds, a substantial portion of shale on the trailing side of the combustion zone can be hot enough to effect retorting of oil shale. This portion which has not been cooled by inlet gas can be part of the heated zone. The heated zone is regarded as that region at a temperature above the retorting temperature of oil shale.
The liquid products and gaseous products of kerogen decomposition are cooled by the cooler oil shale particles in the retort on the advancing side of the retorting zone. A liquid product stream is collected at the bottom of the retort and withdrawn to the surface of the ground through an access tunnel, drift, or shaft. The liquid product stream includes shale oil and water. An off gas containing combustion gas generated in the combustion zone, gaseous products produced in the retorting zone, including hydrocarbons and hydrogen, gas from carbonate decomposition, and the portion of inlet gas that does not take part in the combustion process is also collected at the bottom of the retort and withdrawn to the surface. Such off gas is generally lean, having a relatively low heating value of from about 20 to 100 BTU/SCF and often in the order of about 50 BTU/SCF. The heating value of such off gas from normal retorting operation can be too low for the off gas to be used alone as a fuel gas for establishment of a primary combustion zone or a secondary combustion zone in an in situ oil shale retort. An enrichment operation is, therefore, provided in practice of this invention for producing off gas of enhanced heating value.
In the above-described process, a portion of the fragmented mass can be left unretorted. This can result from gas flow maldistribution through the retort such as channeling of gas flow through the fragmented permeable mass of formation particles in the retort and non-uniform and uneven gas flow through the retort due to non-uniform or uneven void fraction and particle size distribution in the retort. Uneven distribution of void fraction and particle size can occur in an in situ retort because of variations in the blasting technique employed and the amount of explosive used in preparing the in situ retort, as well as physical properties of the formation containing oil shale. Because of such gas flow maldistribution, the front of the retorting zone can be non-uniform. Thus, even when normal retorting is completed, some of the fragmented mass in a portion of the retort can remain unretorted.
Another source of unretorted oil shale in the tract being developed by in situ retorting is formation left unfragmented to function as pillars between in situ retorts. Such pillars can support overburden and serve as barriers to substantial gas flow between fragemented masses in adjacent retorts. The portion of the formation left as pillars can be a significant proportion of the entire formation and can be, for example, approximately 30% of the entire formation in a tract being treated by means of an in situ retorting process. Since the formation present in such pillars has low permeability and low thermal diffusivity, the rate of retorting oil shale in the pillars is slower than in the fragmented permeable mass of formation particles in the retort. Hence, oil shale in the pillars can be left unretorted at the end of normal retorting operation.
Thus, it is desirable to increase the yield or recovery of hydrocarbons from an in situ oil shale retort by recovering carbonaceous values from unretorted oil shale remaining in pillars adjacent the fragmented mass in the retort and from portions of the mass of formation particles containing unretorted oil shale in the retort.
As used herein, normal retorting operation refers to retorting of oil shale in a fragmented permeable mass in an in situ oil shale retort by advancing a heated retorting zone therethrough, with transfer of heat through the fragmented permeable mass being primarily by means of gas flow. Exemplary of the rate of normal retorting operation is an advance of a retorting zone between about one and two feet per day. In one example a retorting zone advances through about 270 feet of fragmented permeable mass in about 165 days of retorting.
As used herein, enrichment operation refers to a period during which there is little, if any, advancement of the retorting zone. As noted hereafter, during an enrichment operation, heat transfer by conduction and radiation are important.